This invention relates to a fuel calibration device for replacing the original fuel metering device for a carburetor. Replacement fuel metering devices have been proposed for example in U.S. Pat. Nos. 3,807,707 and 4,100,663. The fuel insert plat of U.S. Pat. No. 3,807,707 is to be used in conjunction with the original metering block rather than as a substitute for it. The replacement of main jets in U.S. Pat. No. 4,100,663 is accomplished only after modifications are made to the existing fuel bowl, and such modifications are avoided in the present invention.